1. Field of the Invention
The present invention relates to feedback correction for power amplifier linearization.
2. Description of Related Art
There are many methods for extending the upper limit of the power range of an amplifier. These are all intended to compensate for the more-or-less non-linear nature of the amplifier, particularly as the voltage waveform nears the upper and lower limits of the physically-achievable voltage range. Linearity compensation methods fall into two general categories: feed forward and feedback linearization. The methods discussed herein are of the feedback type.
There are two general types of feedback methods for linearization of RF or microwave power amplifiers, namely polar and Cartesian. The polar method of feedback correction compares both the output phase and amplitude to the input phase and amplitude of the amplifier and adjusts (predistorts) the input to account for the distortions introduced by the amplifier. The Cartesian method compares the output in-phase (I) and quadrature (Q) components with the input in-phase and quadrature components and predistorts the input to compensate for the errors introduced by the amplifier. In terms of accuracy, Cartesian feedback offers much higher precision and dynamic range. For instance, it is far easier to maintain linearity in mixers and baseband amplifiers (Cartesian feedback) than in an RF or microwave power detector used to estimate the amplitude of the envelope (polar feedback).
Polar modulation techniques applicable to radiotelephone communications are described, for example, in U.S. Pat. Nos. 6,377,784 and 6,864,668 of the present assignee, entitled HIGH-EFFICIENCY MODULATING RF AMPLIFIER and HIGH-EFFICIENCY AMPLIFIER OUTPUT LEVEL AND BURST CONTROL, respectively, filed Feb. 9, 1999 and incorporated herein by reference. In polar modulation, the phase of the output signal is controlled by modulating the phase of a constant envelope drive signal applied to the amplifier. The amplitude is varied by controlling the DC supply voltage to the power amplifier. Ideally, these DC supply voltage variations would transfer perfectly to the RF envelope, but in practice the conversion is impaired by non-linear effects, characterized as AM-to-AM (amplitude modulation) distortions and AM-to-PM (phase modulation) distortions. Both these types of errors degrade the power spectral density and the EVM (error vector magnitude) of the desired signal.
The accuracy of commonly-used power detectors is inadequate for the precision needed to prevent significant distortions to the spectrum and EVM. For this reason, some circuits have used a Cartesian feedback detector (or IQ demodulator as it is sometimes called).
Unfortunately, the reference for the in-phase and quadrature mixers of a Cartesian feedback detector is not readily available in a polar-modulation-based transmitter. In such a transmitter, the modulated signal does not exist until the fmal amplifier output. At this point the signal has already been impaired by distortions in the amplifier. One possible solution would be to generate another set of signals that includes both amplitude and phase modulation. However, new signals represent a potential for the creation of even more non-linear distortion. Furthermore, in frequency-hopping applications, it would be necessary to hop these references also, adding substantial complexity.
One possibility would be to simply use the I/Q baseband signal (before conversion to polar) as a reference. The approach would then would be to sample the high-power output signal, down-convert it to baseband and compare it to the baseband I/Q reference This approach, however, requires generation of a separate LO frequency which is necessary to downconvert the signal. Moreover, even if such a reference signal were created, it would still not allow for correction of impairments in the power amplifier.